View Full Version : Sticky valve?
11th May 2006, 18:04
I have made various passing remarks about a niggly problem with my ageing GB400. It just rolled 85,000km today and I have had it since new (16 Aug 1988). The main symptoms of the "problem" is that it randomly misfires on startup when idling on choke (I try to keep the revs low) and it backfires (single ear splitter) when throttled off coming to a stop or corner. The misfire "feels" like sudden loss of compresson rather than electrical fault (but wot would I know). Otherwise it goes fine and goes slightly less on a tank of fuel than it did when new.
So I am now wondering if it could be a sticky valve. Any enlightenment would be appreciated:
Is a sticky valve likely/possible?
What would be the symptoms?
Inlet more likely than exhaust?
What are the repair options (other than head off) ie upper cylinder lube (brand etc)?
Does this happen often in bikes?
Add your own question here (with good answer)
11th May 2006, 19:20
Air leak in exhaust:yes:
11th May 2006, 19:46
The main symptoms of the "problem" is that it randomly misfires on startup when idling on choke (I try to keep the revs low) and it backfires (single ear splitter) when throttled off coming to a stop or corner. The misfire "feels" like sudden loss of compresson rather than electrical fault (but wot would I know).
Misfire ? like an after fire ? thats incomplete closure of exhaust valves
Backfire, incomplete closure of inlet valves
And being a Honda of that era and Engine design based on the RFVC XL/XR 500/600s will mean its most likely in need of a valve reset & lap for proper valve contact on the seats.
Check & adjust the valve clearances 1st then go from there if the symptoms continue.
I've been following discusions lately where I'm finding out sticky valves are much more common than we think,they are doing it practicaly all the time.When I get around to it I'll follow up with some tests of my own to prove it for myself.If you watch the KV spikes on a multi cyl running on a 'scope - if you see the spikes fluctuating up and everything else is in order,that's sticky valves.There are methods to prove this,I'll get around to it one day.So I'd run it on a 'scope,and I'd plug my vacuum probe into it so I can see a waveform of the induction...it might pick up the hic cup of the valve.
11th May 2006, 21:39
My suggestion is that you may have a leaking carb boot (the rubber mount between the engine and the carb) or the velocity stack into the airbox(filter) if the bike has one.
11th May 2006, 23:22
other than to repeat others comments. Give her a valve grind and a new set of rings.not too expensive on a single
I do recall when the GB400 was a new model they were having similar problems -might pay to have a yak to ya local honda dealership
12th May 2006, 19:50
Thanks for the comments.
I did previously think that it might be a burnt valve (it did burn one out at around 50000). But compression test now seems OK so I was thinking that it was improperly seating at some spots as the valve rotated. But was talking to mechanically minded workmate who commented about sticking valves so it got me thinking.
It "afterfires" (ie that noise that leaves your ears ringing that eminates from the muffler) any time but certainly when hot. It doesn't seem to misfire much once warm as it doesn't stall. It could be air leaks etc but I have looked for those previously.
I've been following discusions lately where I'm finding out sticky valves are much more common than we think,they are doing it practicaly all the time.
Whoo hoo so there might be something in the sticky valve theory. So, if it might be happening, how can it be fixed (ie RedX, upper cylinder, lump of lead in the tank, magnets????) without ripping the thing open.
4th March 2009, 18:06
I'm having almost the same problem with my GB500.
How did you fix yours?
link to my post: http://www.kiwibiker.co.nz/forums/showthread.php?p=1964951#post1964951
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